Tom explained it in a simple fashion. However, it is more complex than what is seen through simple experimentation. In other words, as long as the car is in motion you will benefit from a film's heat control aspect. When tested, all mannies utilize the same means of extracting the TSER of their film product. The published performance of film products are calculated using a wind speed across the glass of 7mph. Obviously, the faster the vehicle moves, the high the TSER because more absorbed energy is carried away and less will radiate inward to the cabin.
As stated before, 'ceramic' technology in window film relies mostly on absorption, which requires the glass be cooled by wind across its surface. This absorption will allow you to feel what might be radiating off film and glass while driving the car with the AC running.
The one film out there that 'reflects' near infrared (NIR) energy is 3M's Crystalline. Near infrared is the part of the sun's energy that we 'feel' in the immediate when sitting on the protected side of a glass/film barrier. This is because it (NIR) penetrates deeper into the skin, warming moisture surrounding nerve endings and leading the brain to say, warm/hot. Crystalline reflects a very large percentage of NIR (88-97%) in the most intense region of NIR (780- roughly 1200 nanometers). The entire NIR region of the solar spectrum encompasses 780-2500 nanometers.
NIR accounts for approximately 48+% of the sun's energy and visible light (VL) accounts for 49+%, leaving the remaining to ultraviolet (UV). Visible light and UV will create heat once it strikes a surface (such as skin) and is 'felt' at a much slower rate compared with NIR.
So, it's down to absorption versus reflection and it should be seen a slightly higher TSER values.